Lithium-ion battery is a green battery with high energy density, high average output voltage, low self-discharge, no memory effect, wide operating temperature range (is −20° C.˜60° C.), excellent cycling performance, high charge-discharge efficiency, long calendar life and that contains non-toxic and hazardous substances. It relies on the lithium-ions' round-trip of embedding and de-embedding between a negative electrode and a positive electrode to complete battery charging and discharging operations.
While electric vehicles have many advantages comparing to conventional cars, but there still are many problems when the lithium-ion battery packs are applied to the electric cars, the most to be resolved is the group technology of the lithium-ion battery, which involves the following key problems: 1. improving the energy density of the battery pack, 2. keeping the battery pack operating with an appropriate temperature range, 3. ensuring the safety performance of the battery pack.
Energy density of the lithium-ion battery pack is mainly determined by the framework of the battery pack, usually the lithium-ion battery cells are assembled in parallel first, then the parallel modules are assembled in series, and the connections of tabs require the use of the current collector sheets. In the prior art, parallel connections of the cells are the taps of the battery cells directly welded on the current collector sheets, while series connection of the cells is using wires to connect the current collector sheets. However, the battery module has the following disadvantages: 1. When the battery is in the process of operation, it is easy to lose the welding point between the wires and the current collector sheet while shocking, thereby resulting in pseudo soldering and de-soldering; 2. for there are other wires for sampling such as sampling lines exist, then the adding of the wires, makes the internal battery more messy, and takes up too much space.
Patent with application No. CN201010142205.5, titled “battery pack” discloses a battery pack composed of a plurality of soft-package battery cells, which includes connecting means that collect the two batteries, the batteries and the connecting means are collected together by wires, thus the space of the battery pack is occupied greatly or takes a big or extensive real estate, the energy density is reduced, and the welding between the wires and the connecting means is spot welding, it becomes loose by shocking in the operation process, and thus results in pseudo soldering and de-soldering.
Again, among the products the Tesla company put on the market, the battery module of the product with maximum energy density used by the ModelS has the size of length 2.7 m, width 1.5 m, thickness 0.1 m to 0.18 m, is composed of 7600 batteries of 18650, for a 18650 battery cell, the battery shell is approximately 0.02 dm, the electrode sheet's length is 0.65-0.05 after removing the top and bottom members of the battery, therefore the volume of the electrode sheets of the 18650 battery cell is 3.14*((0.18−0.004)/2)2*(0.6)=0.0146 liters, the volume of the electrode sheets used in the batteries of the entire car is 0.0146*7600=111 liters, the volume of the battery module is 27*15*1+3*15*0.8=441 L. In summary, the volume ratio that the electrode sheets occupied the battery pack (Electrode Volume Ratio hereinafter is abbreviated as EVR) is 111/441=25.2%, under the same conditions that the other materials are the same, the higher proportion the electrode sheets take, the higher energy density of the battery pack is, the lower the contrary, 25.2% of EVR for the electric cars has greater constraints to the improvement of the volume energy density, it is not conducive for the arrangement of the power battery to make the mileage of power battery electric vehicles with the indicators of the traditional car in order to meet the needs of customers (the single fuel mileage of the traditional car is almost 600 kilometers, while longest mileage of Tesla is only about 400 kilometers on single charge).
Moreover the battery pack uses cylindrical battery cells, the cylindrical battery cells can be welded to the connecting pieces only by spot welding, and because the contact area is small between the poles of the battery cell and the connecting pieces, so this reduces the current through capacity, also increases the heat, and combining such number of cylindrical battery cells into the battery pack, the heat generated will be very great, the design for heat-dissipation will be more complicated, also more energy will be wasted, these further reduce the effective use of the energy in the overall lithium-ion battery pack.
Lithium-ion battery pack generates heat during charging and discharging process, particularly in fast charging or over-discharging process more serious, if the heat is out of control, the consequences is very serious, decomposition of the electrolyte will occur due to the high temperature, the gases produced will fulfill the cell immediately, the cell will burst when the internal pressure of the cell is too high, at last explosion occurred when it contacts the oxygen in the air. At present, the heat-dissipation and cooling methods for lithium-ion battery pack are mainly air-cooled and liquid-cooled.
Air-cooling method is that through the cold wind poured from the outside into the devices or the fans convection cools the lithium-ion battery pack system. However, since the battery pack is mounted to a group by series and parallel connection, the interval between the battery is small, the battery in the center will have high temperature due to ventilation difficulties, strong wind cools uneven, which causes the decrease in the consistency of the battery, affects the battery life. If for improving the air cooling effect, increasing the flow passage of the cooling air, it will result in a lot of the waste in volume space, and will reduce the volume energy density of the battery pack system. So during the usage of lithium-ion batteries of electric vehicles, the air cooling cannot achieve good cooling effects.
Liquid cooling makes use of the high thermal conductivity of the liquid to take away the heat generated in the lithium-ion battery, in order to achieve the purpose of cooling. Liquid cooling is more uniform and the effect is obvious, especially in less space occupied, is more suitable for electric vehicles with lithium-ion battery pack system. However, at present, the design of liquid cooling system is complex, the design of fluid flow passage is not reasonable, heat-dissipation is not at the main heat-dissipation part of the battery pack. Although also the liquid cooling mode is used, but the actual effect is limited and cannot solve the heat-dissipation problem well.
A patent with application No. 201010619252.4, entitled “Liquid cooling device of power battery pack system” discloses a liquid-cooled apparatus using soft-package lithium-ion batteries as the power battery system, setting the heat transfer structure between the cells first, then they will be connected to form a battery module; then installing the multiple battery modules to the water-cooled base floor with circulating water channels, the outlet of the water-cooled base floor is connected with water pump, the pump is connected via the connecting pipe with an external heat-dissipation device; the inlet of the water-cooled base floor is connected with an external heat-dissipation device. Comparing to the traditional air-cooled technology, the technical solution of the present invention provided here improves cooling efficiency, but does not take full advantage of liquid cooling.
The technical solution here provides the heat transfer plate between the battery cells, the heat generated by the battery cells is exported to the base plate, then through the cooling system provided in the base plate for cooling, in theory, this can indeed achieve uniform heat-dissipation and heat-dissipation effect between the battery packs. But after tests it is found that, when the battery packs are used, the heat-generated parts of the individual are characteristic, not the overall heat production are similar, but concentrate on the tab around, the program aims at thermal conductivity on both sides of the battery cell, but the tab portions that the maximum heat produced are few, this cannot fully utilize the advantages of the liquid cooling. Particularly in extreme cases, the battery after over discharge, or overshoot, the heat capacity produced is great, the prior art technical solution does not solve the heat-dissipation requirements provided under these conditions.